Flexographic printing plates are well known for use in printing on surfaces which are soft and easily deformable, such as packaging materials, e.g. cardboard and plastic films. They can be prepared from a printing plate precursor having a layer consisting of a photo-polymerizable composition, which generally comprises an elastomeric binder, at least one monomer and a photo-initiator. Early patents on flexographic printing plates include U.S. Pat. No. 3,960,572 (ASAHI CHEMICAL), U.S. Pat. No. 3,951,657 (UPJOHN), U.S. Pat. No. 4,323,637 (DU PONT) and U.S. Pat. No. 4,427,759 (DU PONT).
Traditionally, an image is applied to the printing plate precursor by flood exposing the photo-polymerizable layer to actinic radiation (e.g. ultraviolet radiation) with an image mask interposed between the radiation source and the printing plate precursor. The actinic radiation causes polymerization to occur in the areas of the photo-polymerizable layer not shielded by the image mask. After imaging, the plates are processed with a suitable solvent to remove the photo-polymerizable composition in the unexposed areas, thereby creating a relief-based image on the printing plate. The processed plates are then mounted on a printing press, where they are used to transfer ink to a desired printing surface.
The process for manufacturing a flexographic printing plate was simplified by applying a layer for forming the image mask directly on the printing plate precursor. In U.S. Pat. No. 6,521,390 (AGFA), an IR-ablatable layer, substantially opaque to actinic radiation, was laminated on the flexographic printing plate precursor. U.S. Pat. No. 6,358,668 (AGFA) discloses an ink-receiving layer on the photo-polymerizable layer of a flexographic printing plate precursor, wherein the jetted ink on the ink-receiving layer creates an image mask of high optical density. Although the process for preparing the flexographic printing plate was simplified by incorporating an image mask forming layer into the printing plate precursor, the process remains complicated and time-consuming. Furthermore, the process is not environment-friendly due to a high waste production in removing the unexposed areas of the photo-polymerizable layer.
U.S. Pat. No. 5,511,477 (IDANIT TECHNOLOGIES) discloses a method for the production of photopolymeric relief-type printing plates comprising the steps of forming a positive or negative image on a substrate by ink jet printing with a photopolymeric ink composition, optionally preheated to a temperature of about 30 to 260° C.; and of subjecting the resulting printed substrate to UV radiation, thereby curing the ink composition forming the image. Suitable substrates for this method are restricted to steel, polyester and other rigid materials, limiting the possibilities for flexographic applications. Another problem is that jetted droplets of the polymeric ink are still mobile and tend to deform, thereby preventing accurate reproduction of small dots and preventing the formation of sharp edges and hence the formation of a sharp image.
U.S. Pat. No. 6,520,084 (CREO) discloses a method for manufacturing a flexographic printing plate by means of multiple passes of an ink-jet unit employing two different elastomers that are deposited on a modifying surface. For jetting, the elastomers can be liquefied by heating meltable polymers to temperatures between 100 and 150° C. or by dissolving them in hazardous and toxic solvents such as toluene. The requirement of high temperatures restricts not only the choice of suitable substrates, but also limits the ink-jet printer to a “solid ink-jet” device. The toluene is allowed to evaporate between every two deposited layers, creating a hostile environment.
EP 1428666 A (AGFA) discloses a method for preparing a flexographic printing plate by jetting radiation curable ink-jet ink on a resilient substrate. The disclosed inks do not contain any elastomers and the quality of the flexographic printing plate is inferior to conventional flexographic printing plates. Experiments by the present inventors to prepare a curable jettable liquid containing elastomers in a sufficient amount to improve the quality of such flexographic printing plates were not successful.
There is therefore a need to provide an improved curable jettable liquid for manufacturing a flexographic printing plate with a high image quality and applicable to a wide range of applications, including printing on soft and easily deformable surfaces. Furthermore, a large choice of suitable, cheap ink-jet printers is desirable, i.e. printers using print heads of a continuous type or print heads of a piezoelectric, a thermal, an electrostatic and an acoustic drop on demand type.